PURPOSE: To provide the welding structure of a header part with high reliability by suppressing material strength from being deteriorated by decreasing the width (thickness) of a welding heat influence part formed on a stub tube as far as possible, and forming the welding structure in which stress can be prevented from being concentrated on a part to be welded.

CONSTITUTION: Temperature increase by welding can be suppressed and the width (thickness) of a heat influence part is reduced by increasing the thickness of the stub tube 1a at the part to be welded on a header. Also, the terminal part of the stub tube 1a is formed so as to weld by inserting to the recessed part of an open tip working part provided at a header side, which provides the welding structure of the header part in which no crack is generated even when an external force of curvature is applied to the part to be welded. In this way, it is possible to improve soundness and reliability for a boiler and every kind of heat exchanger used in high temperature and high pressure.